Drawing Enantiomers

PROBLEM 20.12 Which of the following complexes can exist as enantiomers Draw the structure of each enantiomer. 

The antibiotic linezolid is the pure 5 enantiomer. Draw the structural formula of the molecule with this configuration. 

To show that (/f)-2-butyl (/f,/f)-tartrate and (S)-2-butyl (/ ,/f)-tartrate are not enantiomers, draw and name the mirror images of these compounds. 

Describe what is meant by a pair of enantiomers. Draw an example of a pair of enantiomers. 

Because you have been asked to draw the R enantiomer, draw an arrow clockwise from the group with the highest priority to the next available bond and put the group with the next highest priority on that bond. 

When 2-methyl-2-butene is treated with NBS and irradiated with UV light, five different monobromination products are obtained, one of which is a racemic mixture of enantiomers. Draw all five monobromination products and identify the product that is obtained as a racemic mixture. 

Let s return to bromochlorofluoromethane as a simple example of a chiral molecule. The two enantiomers of BrCIFCH are shown as ball-and-stick models, as wedge-and-dash drawings, and as Fischer projections in Figure 7.6. Fischer projections are always generated the same way the molecule is oriented so that the vertical bonds at the chirality center are directed away from you and the horizontal bonds point toward you. A projection of the bonds onto the page is a cross. The chirality center lies at the center of the cross but is not explicitly shown. 

FIGURE 7.6 Ball-and-spoke models (/eft), wedge-and-dash drawings (center), and Fischer projections (right) of the R and S enantiomers of bromochlorofluoromethane. 

C atoms are labelled a-e (see text), (b), (c) Line drawings of the two enantiomers of C76 viewed along the short C2 rotation axis and illustrating the chiral D2 symmetry of the molecule. 

Next, examine the lowest-unoccupied molecular orbital (LUMO) for the cation. The components of the LUMO (its lobes ) identify locations where the cation might bond to a water molecule. How many lobes are associated with C 7 For each lobe, draw the alcohol that will be produced (show stereochemistry). How many alcohol enantiomers will form If more than one is expected, decide which wiU form more rapidly based on the relative sizes of the lobes. 

Problem 9.3. Alanine, an amino acid found in proteins, is chiral. Draw the two enantiomers of ala- nine using the standard convention of solid, wedged, and dashed lines. 

Drawing the Three-Dimensional Structure of a Specific Enantiomer... 

Draw tetrahedral representations of the two enantiomers of the amino acid cysteine, HSCfi CHfNH lCX H, and identify each as R or S. 

Draw the R enantiomer of the reactant, and then change the configuration of the chirality center while replacing the Br with a CN. 

Imagine that the two brothers are twins. They are identical in every way except one. One of them has a mole on his right cheek, and the other has a mole on his left cheek. This allows you to distinguish them from each other. They are mirror images of each other, but they don t look exactly the same (one cannot be superimposed on top of the other). It is very important to be able to see the relationship between different compounds. It is important to be able to draw enantiomers. Later in the course, you will see reactions where a stereocenter is created and both enantiomers are formed. To predict the products, you must be able to draw both enantiomers. In this section, we will see how to draw enantiomers. 

So we must learn how to draw one enantiomer when we are given the other. When we see the different ways of doing this, we will begin to recognize when compounds are enantiomers and when they are not. 

The simplest way to draw an enantiomer is to redraw the carbon skeleton, but invert all stereocenters. In other words, change all dashes into wedges and change all wedges into dashes. Eor example,... 

The compound above has a stereocenter (what is the configuration ). If we wanted to draw the enantiomer, we would redraw the compound, but we would turn the wedge into a dash ... 

This is a pretty simple procedure for drawing enantiomers. It works for compounds with many stereocenters just as easily. For example,... 

PROBLEMS Draw the enantiomer of each of the following compounds. 

There is another way to draw enantiomers, hi the previous method, we placed an imaginary mirror behind the compound, and we looked into that mirror to see the reflection. In the second method for drawing enantiomers, we place the imaginary mirror on the side of the, compound, and we look into the mirror to see the reflection. Let s see an example ... 

But why do we need a second way of drawing enantiomers Didn t the first method seem good enough The first method (switching all dashes with wedges) was pretty simple to do. But there are times when the first method will not work so well. There are a few examples of cyclic and bicycfic carbon skeletons where the wedges and... 

When dealing with these kinds of compounds, it is much easier to use the second method to draw enantiomers. Of course, if you like this method, you can always use this second method for all compounds (even those that show wedges and dashes). 

Answer This is a rigid bicyclic system, and the dashes and wedges are not shown. Therefore, we will use the second method for drawing enantiomers. We will place the mirror on the side of the compound, and draw what would appear in the mirror ... 

In all of our examples so far, we have been comparing two compounds that are mirror images. For them to be mirror images, they need to have different configurations for every single stereocenter. Remember that our first method for drawing enantiomers was to switch all wedges with dashes. For the two compounds to be enantiomers, every stereocenter had to be inverted. But what happens if we have many stereocenters and we only invert some of them ... 

It is possible for a componnd to be its own mirror image. In such a case, the compound will not have a twin. It will be all by itself, and the total number of stereoisomers will be an odd number, rather than an even number. That one lonely compound is called a meso componnd. If you try to draw the enantiomer (using either of the methods we have seen), yon will find fhat your drawing will be the same compound as what yon started with. 

If a molecule has an internal plane of symmetry, then it is a meso compound. If yon try to draw the enantiomer (nsing either one of the two methods we saw), you will... 

There is one fail-safe way to tell if a compound is a meso compound simply draw what you think should be the enantiomer and see if you can rotate the new drawing in any way to superimpose on the original drawing. If you can, then the compound will be meso. If not, then your second drawing is the enantiomer of the original compound. 

Answer We need to try to draw the mirror image and see if it is j ust the same compound redrawn. If we use the second method for drawing enantiomers (placing the mirror on the side), then we wiU be able to see that the compound we would draw is the same thing ... 

Now we can understand why we cannot draw a Fischer projection sideways. If we did, we would be inverting the stereocenter. To draw the enantiomer of a Fischer projection, do not turn the drawing sideways. Instead, you should use the second method we saw for drawing enantiomers (place the mirror on the side of the compound and draw the reflection). Recall that this was the method that we used for drawings where wedges and dashes were implied but not shown. Fischer projections are another example of drawings that fit this criterion ... 

EXERCISE 7.75 Determine the configuration of the stereocenter below. Then draw the enantiomer. 

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